Random FD aperture ring question

This is a pretty obscure question, but this is a helpful and knowledgeable group so I'll give it a try. I was just at my in-laws, and it turns out my father-in-law had Canon FDn 28/2.8 and FDn 50/1.2 (the non-L version) lenses that he generously gave to me (plus a beautiful but at best semi-functional Konica IIIM from 1959 I will see if my local camera repair place can fix up). I have an AE-1 Program already for FD lenses, and decided to pick up a Fotodiox adapter to use the 50 with m4/3 since I don't have a 1.2 or 1.4 lens. Overall it seems to be working great on my E-M1.

Here's the question. The aperture ring has markings for 1.2 and 1.4 - see photos below - but the aperture itself doesn't seem to stop down or open up at all when I move between the two (attached to the adapter so the aperture ring works); I only see the aperture blades moving when I click to just past 1.4 to what I assume is 1.8 or so (from there on everything is as expected). What I can't figure out is whether it's at 1.2 where it says both 1.2 and 1.4, or at 1.4 for both. On the E-M1, there is no change in exposure when I move the ring between the two, either, but since it's an adapted lens it doesn't say what the aperture value is.

Am I missing something obvious? Is the white line actually a hard stop or something rather than indicating the f-stop? Do older lenses sometimes have imprecise f-stop markings? Is there something (slightly) wrong with the lens? I know none of this matters much but I find it curious. Thanks!

Aperture ring at 1.2:{}

Aperture ring at 1.4 but apparently identical actual aperture opening to when it's at 1.2:{}

Yes, it's locked. And rotating it from 1.2 to 1.4 doesn't seem to close it down in any way. It visibly closes down once moved just past 1.4 but not at all between 1.2 and 1.4, which is what I find surprising.

The difference between f/1.2 and f/1.4 is rather small. It's only 1/3 stop so the blade movement will be small and might not be visible from the front. The OP should check from the rear and with their eye about 1" from the rear of the adapter. A good second check would be to mount the lens on his AE-1 and check it there. It is possible that the adapter is failing to open the blade quite all the way leaving the f/1.2 setting not fully open and matching the f/1.4 setting.

With my FD lenses, they can be really picky as to whether you've got them correctly locked on the adapter. If not, you can get anything between the aperture ring not doing anything at all, through to it producing a little bit of aperture movement but not over the full range. I was using my 300mm f4 last week and the CA was so bad I could see it in the EVF whilst shooting! When I stopped down, no effect, then I realised the shutter speed wasn't changing, then I realised the lock ring wasn't quite fully engaged...

Thanks all - I appreciate the thoughts and tried various things out. I ended up mounting my 50/1.8 on my E-M1, having it meter a scene, then comparing that same scene to the E-M1 with the 50/1.2. If I did my math right, the 50/1.8 at 1.8 was exactly 1 stop slower than the 50/1.2 at both its 1.2 and (ostensible) 1.4 settings (which were identical). The 50/1.2 gave the same shutter speed when at the f/2 marking as the 50/1.8 at 1.8, and the 50/1.2 at its 1.8 marking was metering what 1.4 would roughly be based on the 50/1.8 at 1.8. If I calculated things right, for some reason the 50/1.2 aperture markings seem to be slightly "off" compared to the 50/1.8. Or maybe the adapter is doing something weird, while still letting in a ton of light. But in any case, it looks like I got a free lens that seems to work at 1.2, and is fun to play with (though definitely a different look - esp. wide open - than the modern m4/3 lenses give).

...
Brightness can easily be determined by looking at the RGB values of the same item in both shots. Make sure it's set to manual not anything else.

That's the logical proof

Click to expand...

+1

the camera's display is rounded off using rather course increments making precision impossible. The best approach is to shoot a light grey image in manual (manual f/stop, manual shutter speed, and manual ISO) and sample the density values in a good image editor. Be sure to use a very steady light source (read: daylight) or use a slow shutter speed (longer than 1/30th) if using artificial light to avoid slight variances in exposure cause by varying brightness of the light source.

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